1. Field of the Invention
The present invention relates to a spindle motor startup method, and more particularly to a startup method for spindle motors prone to torque balance conditions, such as where the ratio of the number of rotor magnetic poles to the number of stator slots is 4:3.
2. Background Art
Conventional 3.5" hard disk drives (HDD) having rotational speeds of at least 7200 rpm are typically equipped with a spindle motor such as an eight-pole twelve-slot spindle motor having a rotor magnetic pole to stator slot ratio of 2:3 for optimum startup characteristics. Alternatively, such disk drives may have a spindle motor like an eight-pole nine-slot spindle motor that has a construction to break a torque balance. Startup malfunctions caused by a magnetic balance between a rotor and a stator seldom occur in these types of motors. Recently, three-phase spindle motors such as a twelve-pole nine-slot spindle motor have started to be commonly used in disk drives. In these drives the ratio of the number of rotor magnetic poles to stator slots is 4:3.
Spindle motors with a 4:3 ratio of rotor magnetic poles to stator slots, like the twelve-pole nine-slot spindle motor described above, are prone to startup problems because the initial starting-torque capacity may not be adequate to overcome a magnetic balance that is created in certain startup positions of the rotor and stator. Such startup malfunctions result in long startup times for the motors, which is a critical parameter in applications such as disk drives.
FIGS. 5A through 5G show the rotor position in a four-pole three-slot spindle motor as a function of time in a startup malfunction. FIG. 5A shows the initial rotor position, and FIGS. 5B-5G sequentially show the motor at subsequent times. The four-pole three-slot spindle motor 51 comprises a cylindrical rotor 52 comprising four poles composed of pairs of the north and south poles, and a stator 53 composed of three slots, that is, U, V, and W phases. Furthermore, as shown in FIGS. 5A through 5G, a driver not shown switches current for starting the motor in the order of the U phase to the V phase, the U to the W, the V to the U, the W to the U, and the W to the V. At this time, the startup malfunction is shown as a state of not being able to normally start the rotation of the motor with sequentially repeating reverse rotation and normal rotation as shown in FIGS. 5A through 5G. This is because rotor 52 and stator 53 are magnetically balanced due to relative positions of rotor 52 and stator 53 at the time the motor stopped as shown in FIG. 5A, and the magnetic circuit cannot generate sufficient initial starting-torque to break the balance.
An object of the present invention is to provide a spindle motor apparatus and a startup method that can provide excellent startup performance even in spindle motor configurations that are prone to startup malfunctions, such as where the ratio of rotor magnetic poles to stator slots is 4:3.